1. Field of the Invention
The present invention relates to an apparatus and a method for simultaneously supporting a Multiple-Input Multiple-Output (MIMO) and beamforming in a wireless communication system. More particularly, the present invention relates to an apparatus and a method for transmitting a data stream using an improved Space-Time Coding (STC) and BeamForming (BF) technique.
2. Description of the Related Art
With the rapid development of the wireless communication market, providing various multimedia services under a wireless environment is required. Recently, as greater capacity of transmission data and higher speed in data transmission are supported in order to provide such multimedia services, extensive research is being conducted in a multiple antenna scheme capable of efficiently using the limited frequency bands.
A wireless communication scheme may be divided into a Single-Input Single-Output (SISO) scheme and an MIMO scheme depending on the number of antennas used during transmission/reception. The SISO scheme performs transmission/reception via one antenna, and the MIMO scheme performs transmission/reception via a plurality of antennas. The MIMO scheme may be roughly divided into three techniques: a transmit diversity technique (referred to as a ‘Space-Time Coding (STC) technique’ hereinafter) that can obtain a diversity gain using a plurality of space paths formed by a plurality of antennas; a spatial multiplexing technique that can obtain a multiplexing gain by simultaneously transmitting a plurality of data via a plurality of antennas; and, a BeamForming (BF) technique that can gain an array gain by forming a beam suitable for a channel using a plurality of antennas and performing transmission.
The BF technique forms a beam in the direction of each terminal by multiplying a transmission signal transmitted to each terminal by a specific beamforming coefficient, and transmitting the same via a plurality of antennas. When the BF technique is applied to a system, a reception Signal to Noise Ratio (SNR) of a terminal increases and a fading influence by a wireless channel reduces, so that a reception performance of the system can be improved. However, to apply the BF technique and achieve the best performance improvement, a beamforming coefficient and a transmission channel need to match, and when an error between a beamforming coefficient and a transmission channel occurs, any gain that may be obtained by the BF technique drastically reduces. More particularly, in the case where a velocity of a terminal is high in an actual operation, when the BF technique is applied to a system, an error between a beamforming coefficient and a transmission channel increases even more and thus it is difficult to expect any improvement in system performance.
By combining the BF technique with at least one other technique, such as the SISO scheme, the STC technique, or the spatial multiplexing technique, a new type of BF technique may be applied to the system. For example, a Single-Input Single-Output and BeamForming (SISO BF) technique which combines the SISO scheme with the BF technique, a Space-Time Coding and BeamForming (STC BF) technique which combines the STC technique with the BF technique, etc. may be applied to the system. A method for determining a beamforming coefficient may differ depending on an antenna shape of each technique.
Assuming a four transmission antennas example, a system that applies the conventional STC BF technique has a (2+2)-structure. The (2+2)-structure denotes a structure that transmits a first data stream of two data streams via antennas 1 and 2, and transmits a second data stream via antennas 3 and 4. A structure of a system that applies the STC BF technique is not simply generalized using N transmission antennas, that is, the structure is not generalized as an (N+N)-structure. Therefore, an STC BF technique having a structure that can be simply generalized using N transmission antennas is required.